In recent years an increasing number of patients have been suffering from circulatory system related diseases, including ischemic diseases such as myocardial and cerebral infarction. In order to prevent such diseases it is important to detect symptoms of arteriosclerosis and administer treatment at an early stage.
Thickness of an intima-media complex in the carotid artery (herein, referred to as intima-media thickness or abbreviated as IMT) is attracting attention as an indicator which can be used in diagnosis of arteriosclerosis. IMT is an important indicator of early-stage atherosclerosis in the carotid artery.
FIG. 14 is a cross-sectional diagram illustrating a cross-section of the carotid artery in a direction in which the carotid artery extends (herein, referred to as a longitudinal cross-section). The carotid artery is a blood vessel that consists of a vascular wall 201 and a lumen 204. The vascular wall 201 is composed of a tunica intima 202, a tunica media 203, and a tunica adventitia 205 in respective order in a direction outwards from the lumen 204. The term IMT refers to thickness of an intima-media complex 206, which is a complex of the tunica intima 202 and the tunica media 203. An ultrasound diagnostic device can be used in order to visualize the intima-media complex 206 between the lumen 204 and the tunica adventitia 205.
Ultrasound examination can be performed simply and in a non-invasive manner, and therefore is commonly used as a method for measuring IMT. In IMT measurement it is advantageous to use the carotid artery as a measurement target in terms that the carotid artery is particularly susceptible to arteriosclerosis, and also in terms that the carotid artery is located at a relatively shallow depth approximately 2 cm to 3 cm under the skin surface, enabling simple measurement by ultrasound. Measurement of IMT is typically performed based on a two dimensional (2D) image which is an ultrasound diagnostic image orientated along the longitudinal direction of the carotid artery. More specifically, IMT can be measured by detecting an interface between the lumen 204 and the tunica intima 202 illustrated in FIG. 14 (herein, referred to as a lumen-intima interface 207), and an interface between the tunica media 203 and the tunica adventitia 205 illustrated in FIG. 14 (herein, referred to as a media-adventitia interface 208).
FIG. 15 is a perspective diagram illustrating structure of the carotid artery in terms of the longitudinal direction. As illustrated in FIG. 15, the carotid artery is configured by a common carotid artery (herein, abbreviated as “CCA”) 213 located at a central end of the carotid artery, and an internal carotid artery (herein, abbreviated as “ICA”) 215 and an external carotid artery (herein, abbreviated as “ECA”) 216, each located at a peripheral end of the carotid artery. The carotid artery is also configured by a bulb of the common carotid artery (herein, abbreviated as “bulb”) 214 which is located between the CCA 213 and each of the ICA 215 and the ECA 216. A bifurcation of the common carotid artery (herein, abbreviated as “bif”) 217 is located at a branching point from the bulb 214 to the ICA 215 and the ECA 216.
FIG. 16 is a schematic diagram illustrating a 2D image of the carotid artery orientated along the longitudinal direction. As illustrated in FIG. 16, when an ultrasound diagnostic device is used to measure an IMT measurement range 212, a region of interest (herein, abbreviated as “ROI”) 211 is determined which either spans across a vascular wall which is relatively far from the skin surface (herein, referred to as far-side wall 209) or a vascular wall which is relatively close to the skin surface (herein, referred to as near-side wall 210). FIG. 16 illustrates an example in which the ROI 211 is determined such as to span across the far-side wall 209. A section of vascular wall included within the ROI 211 is defined as the IMT measurement range 212, and a value indicating IMT is measured for example as a maximum value of IMT (herein, referred to as max. IMT) or a mean value of IMT (herein, referred to as mean IMT) measured in the IMT measurement range 212. With regards to the IMT measurement range 212, Non-Patent Literature 1 recommends that the IMT measurement range 212 is set as a range which extends for 1 cm towards the CCA 213, starting from a boundary 219 between the CCA 213 and the bulb 214 (herein, referred to as a CCA-bulb boundary 219).
Various methods have been proposed for enabling simple IMT measurement by simplifying a complicated operation of determining the ROI 211, which is used to define the IMT measurement range 212. For example, Non-Patent Literature 2 proposes a method for setting an IMT measurement range in which an inflection point at the CCA-bulb boundary 219 between the CCA 213 and the bulb 214 is detected, and an IMT measurement range is set using the inflection point as a reference.